Over the weekend, Clinton won two more Democratic contests that drew her within shouting distance of clinching the nomination — the Virgin Islands and (more important) Puerto Rico. On Monday morning, she needed 23 moredelegates to hit the 2,383-delegate mark that constitutes a majority for the convention. She had 1,812 pledged delegates and commitments from more than 500 superdelegates, but she was a bit short of the threshold.

What happened on Monday was that the Associated Press received commitments from enough superdelegates — just enough superdelegates, in fact — to put Clinton over the top. With 23 more superdelegates in hand, Clinton reached the 2,383 mark, and the AP made the call.

Understandably, that out-of-the-blue determination prompted even more frustration and confusion than was already expected. How was the AP sure that those superdelegates wouldn't change their minds? Why release new totals the day before the critical primaries in California? Who were the superdelegates that made the difference?

We did our best to answer those questions.How does the AP know the superdelegates won't change their minds?

Overnight, the Associated Press published an explanation of how it collects those commitments. It's worth excerpting at length:

The AP surveys the superdelegates throughout the primary season to track whom they plan to support at the July convention.

If a superdelegate tells the AP he or she plans to unequivocally support a candidate at the convention, that's added to the candidate's tally.

Those who decline to answer, say they have yet to make a decision or express any reservations are listed as uncommitted.

What's important to remember is who these superdelegates are. Pew Research broke down their demographics earlier this year. More than half of the 714 superdelegates are members of the Democratic National Committee. Another 261 are members of Congress or governors.